1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and particularly, for manufacturing a hetero bipolar transistor (HBT) having a wide energy band gap emitter layer made of silicon carbide (SiC).
A bipolar transistor (BPT) is one of the basic elements of a large scale integrated circuit (LSI), and the performance of BPTs in LSIs, such as operation speed and packing density of BPTs has been increased by miniaturizing the size and decreasing the parasitic resistance and capacitance of BPT. During the development, it has been considered that HBT having a wide energy band gap emitter would be more likely to increase performance. However, a stable production of HBTs has not been realized on a silicon (Si) substrate.
2. Description of the Related Art
The inventors of the present invention and others have studied whether SiC is useful for a wide energy band gap emitter of a HBT and have experimentally succeeded forming a wide energy band gap SiC emitter layer on an Si substrate. The result of the study was published by T. sugii, T. Ito, Y. Furumura, M. Doki, F. Mieno and M. Maeda in 1987, under the title of "Si Heterojunction Bipolar Transistors with Single-Crystalline .beta.-SiC Emitter" in J. Electrochem. Soc., Vol. 134, pp 2545-2549, in 1987.
Usually in an LSI, the HBT has planar structure, same as regular BPTs. However, a wide energy band gap SiC emitter layer has a problem in the fabricating process thereof. That is, when an SiC emitter layer is formed on a silicon dioxide (SiO.sub.2) layer by a chemical vapor deposition (CVD) method in the atmosphere of a reactant gas consisting of methane (CH.sub.4) and trichlorosilane (SiHCl.sub.3), a part of thc SiO.sub.2 layer is deoxidized by CH.sub.4, deteriorating an insulation characteristic of the SiO.sub.2 layer. Hereupon, the SiC emitter layer is formed by the CVD method with the following chemical reaction: EQU SiHCl.sub.3 +CH.sub.4 .fwdarw.SiC+HCl, (1000.degree. C.),
wherein, HCl is hydrogen chloride and 1000 .degree. C. is a temperature at which the reaction is advanced.
Furthermore, the above reaction is usually performed in a carrier gas of hydrogen (H.sub.2) for preventing Si from being oxided. FIG. 1 is a cross sectional view of an SiC layer 6 formed on an Si substrate 1 through an SiO.sub.2 layer 3 and is for illustrating a formed result of the SiC layer 31 due to the prior art method of forming an SiC layer. In FIG. 1, the SiO.sub.2 layer 3 formed on the Si substrate 1 is partially etched to provide an active region 21 in an aperture 51. When the SiC layer 6 is grown on the active region 21, methane (CH.sub.4) reacts with a part of the SiO.sub.2 layer 3, so that the part is deoxidized, causing defects that degrade the insulating characteristic of the SiO.sub.2 layer 3. In particular, this reduction occurs in the ambient gas of H.sub.2. Because of this, it has been very hard to realize a stable production of HBTs having planer structure with a wide energy band gap SiC emitter layer.